This adds branch name to the default URLs for IPA images, ensuring that
devstack runs will use the correct branch of IPA for tests.
It introduces a variable IPA_DOWNLOAD_BRANCH to accomplish this, which
defaults to master. This default will be set to the branch name on
stable branches, e.g. stable/mitaka. It is modified to match our URL
scheme for agent images, e.g. stable-mitaka.
Change-Id: I3ec6dafb9522b5fe79e0c6681c2578d2a723380a
Closes-Bug: #1561756